In gas lift production operations, one or more tubing strings may be disposed within a well casing, each tubing string servicing a different producing zone. At various levels within the well, gas lift valves are located in each of the tubing strings and introduce gas into the tubing strings to provide a lifting function that lifts production fluid such as water and oil, from the producing zone to the surface of the earth where it may be carried away by appropriate piping. Gas lift valve mechanisms generally take the form of mandrels that are connected into and form a part of the tubing string and these mandrels are fairly short in length, being in the order to 3 to 6 feet.
The production life of gas lift valve mechanisms has been found to vary considerably because the well enivornment in which the gas lift valves are located may be of quite hostile nature. When a gas lift valve at any particular level within the well ceases to function properly, production of produced fluid may be severely reduced or perhaps terminated entirely. When a gas lift valve malfunctions, it is typically necessary to remove the tubing string from the well in order to replace or repair the defective gas lift valve and, at times, the tubing is pulled only after a number of gas lift valves within the well cease to function. Obviously, poor production can be expected if one or more of the gas lift valves have become defective.
Even though gas lift valves may continue to function normally, removal of tubing strings and replacement of gas lift valves is sometimes necessary due to changing well conditions. During the production life of a well, there may be a great change in the character of the fluid that is produced and at times it is necessary to provide gas lift valve mechanisms that are differently calibrated in order to satisfy the changing production requirements.
Where a well may be provided with two or more tubing strings for producing well fluid from two or more different production formations, introduction of gas into the annulus between the casing and the tubing strings must be discontinued in most cases in order to allow one of the tubing strings to be removed from the well for repair or replacement of gas lift valves. Even though the remaining tubing strings are in condition for efficient production, production must nevertheless be ceased for the total period of time necessary for servicing, thereby adding to the cost of servicing the value of losses in production.
Removing tubing strings from a well casing for repair or replacement is typically a very expensive, time consuming operation which involves utilization of costly equipment such as a well servicing rig plus its crew resulting in a very high cost per hour. For example, pulling the production tubing for servicing or recalibration may result in a servicing cost of $100,000 or more. Other factors that influence the cost of pulling tubing strings for repair concern equipment that is typically replaced when such well servicing operations are conducted. For example, packer devices must be broken loose between the casing and tubing and removed from the well and it is often necessary to replace such packer devices with new equipment when reinstalling the tubing within the well. It may also be necessary to remove more than one tubing string from the well even when servicing is required for a single tubing string of a multiple completion well. The magnitude of these servicing operations is of course, undesirable both from the standpoint of the downtime or loss of production involved and because of the actual expense of the servicing operation.
It is considered desirable to provide wells that are produced by a gas lift operation with means for preventing the necessity for immediate pulling of a tubing string and conducting major servicing operations when it is determined that one or more gas lift valve mechanisms may have malfunctioned.